'Mountains' on stars could trigger gravitational waves

Neutron stars - not just rocky planets and moons - can boast topographical features such as plateaus or mountains, a new computer simulation suggests. As the stars rotate, these structures should ripple the surrounding fabric of space, producing gravitational waves that astronomers have long hoped to detect.

Einstein's theory of general relativity predicts that the motion of slightly lopsided or asymmetrical objects should trigger gravitational waves in space. But so far detectors set up to capture the waves - such as LIGO (Laser Interferometer Gravitational-Wave Observatory) in the US and Virgo in Italy - have failed to find any sign of them.

Now, Matthias Vigelius and Andrew Melatos, both of the University of Melbourne in Australia, provide new hope that detectable waves may be produced by some neutron stars.

Neutron stars are the cores left behind when relatively massive stars explode as supernovae. They are incredibly dense, packing about as much mass as the Sun into a sphere just 20 kilometres or so across, and some rotate hundreds of times per second.

Because of their extreme gravity and rotational speed, neutron stars could potentially make large ripples in the fabric of space - but only if their surfaces contain bumps or other imperfections.

Jelly-like consistency

Indeed, Vigelius and Melatos found that massive, stable 'mountains' can grow on the surface of a neutron star using material stolen from an ordinary companion star.

In the first 3D computer simulation of this process, they found that the neutron star's magnetic field channels this stolen matter to its magnetic poles, creating a mountain on each pole.

The magnetic field reinforces the mountains, preventing them from being flattened down completely by the star's powerful gravity. Mountains with about as much mass as the planet Saturn can form and persist at each pole, according to the new simulation.

The matter pulled off the companion star would start out as a gas of protons and electrons. But under the high-pressure conditions at the neutron star's surface, it would be transformed into a material made of pure neutrons, which some scientists believe would have a jelly-like consistency.

What would the mountains look like up close? Even with the support of the magnetic field, they would be very flattened compared to their counterparts on Earth. They would extend about 3 kilometres horizontally, but only rise to between 10 centimetres and 1 metre above the surrounding surface, Vigelius says.

Stable signal

Because they are on the surface of a star, they would also be very hot. "Their colour would be a nice hue of X-rays, so it would in fact not be advisable to have a close-up look!" Vigelius told New Scientist.

Despite having a low profile, the mountains would be large enough to produce strong gravitational waves as they are carried around by the star's rotation.

That's because observations have shown that neutron stars' magnetic poles and rotational poles often do not coincide. The mountains would thus move in circles as the star rotates rather than simply staying put on the rotational poles, and that asymmetry would set off the ripples in space.

In some ways, these waves would be easier to find than those from one-off events like the merger of neutron stars or black holes, which would only last a minute or so and might give a complicated signal.

Neutron star mountains, by contrast, would give off a very regular pattern of waves that would continue essentially indefinitely, making them easier to distinguish from random noise in the detectors, the researchers say.

Smooth surface

Vigelius and Melatos believe that such waves could even be observable by the existing LIGO detector, depending on how massive the mountains on nearby neutron stars have grown. But actually finding any such signals in the detector data might require more computing power than LIGO currently has available - a problem that a distributed computing programme called Einstein@home hopes to remedy.

Benjamin Owen, a member of the LIGO team at Pennsylvania State University in University Park, US, says most scientists think a merger of neutron stars or black holes - which produce more powerful waves - will produce the first detectable gravitational wave signal.

But he says a recent search for gravitational waves from the nearby neutron star in the Crab Nebula is the first gravitational wave search to return really interesting information.

The fact that it did not detect any gravitational waves allowed scientists to conclude that the neutron star is very smooth. Any deviation from perfect roundness - due to mountains, for example - must amount to no more than a few tenths of a percent.

Although some uncertainty remains in the size of neutron star mountains, the simulation by Vigelius and Melatos is a good step forward, Owen says. "This is definitely nice progress firming things up," he told New Scientist.

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An illustration shows a neutron star with a single mountain (yellow) covering much of its surface and another mostly out of view on the star's other magnetic pole. Magnetic field lines are shown in blue (Illustration: M Vigelius/A Melatos/University of Melbourne)